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EXTRA3 Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Succession is a continuous change in the species composition, structure, and function of a forest through time following disturbances.

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Forest Succession
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Succession is a continuous change in the species composition, structure, and function of a forest through time following disturbance. Each stage of succession is referred to as a successional sere. The final stage of succession, which is generally self-replacing, is referred to as the climax sere. There are two major types of succession: primary and secondary. Primary succession is the establishment of vegetation on bare rocks or radically disturbed soil. Secondary succession is the reestablishment of vegetation following a disturbance that killed or removed the vegetation but did not greatly affect the soil. Volcanic eruptions, retreating glaciers, and bare sand dunes are examples of sites subject to primary succession, while clear-cutting of forests, wild fires, and hurricanes are examples of sites subject to secondary succession. Hundreds to thousands of years are required for primary succession to reach the climax sere, compared to decades to hundreds of years for it to occur in secondary succession. A longer time is needed to reach the climax sere for primary than secondary succession because soil development must first take place in primary succession. The rate of succession is dependent upon the extent of the disturbance and the availability of appropriate seeds for recolonization.

What morphological (structural) and ecophysiological characteristics determine the species composition and abundance in succession? In general, nitrogen fixing plants(plants that can make use of atmospheric nitrogen) are important early succession species in primary succession because nitrogen is not derived from the weathering of rock and little or no organic matter is present in the soil. Weedy plants are common early successional species because of their rapid growth and high reproductive rates, while stress-tolerant species are common late successional species.

The structure of a forest changes as well in secondary succession. Depending on the type and the severity of the disturbance, a moderate to large amount of dead organic matter from the previous forest remains on the site immediately from the disturbance. The leaf area of the forest is at a minimum and slowly increases as new vegetation occupies the site. Following a disturbance, such as a fire, the new canopy (the uppermost spreading and branching layer of a forest) is largely composed of similar-aged, or even-aged, trees. Light, nutrient, and water availability are highest during the early successional sere because the vegetation has not completely occupied the site. Canopy closure, or maximum leaf area, can occur within several years after disturbance in some tropical forests, but may take three to fifty years in evergreen forests.

In the second stage of forest development there is tree mortality caused by competition for light, nutrients and water. The intense intraspecies (within a species) and interspecies (between species) competition for light, nutrients and water induces the mortality of plants that are shaded or have one or more life-history characteristics that are not well adapted to the changing environment. The third stage of forest development is characterized by openings in the overstory canopy, caused by tree mortality, and the renewed growth of understory in response to increased light reaching the forest floor. Consequently, the forest canopy becomes more complex, or multilayered. The final stage of forest development, the climax or old growth stage, is characterized by a species composition that in theory can continue to replace itself unless a catastrophic disturbance occurs. Unique characteristics of old growth forests include large accumulation of standing and fallen dead trees--referred to as coarse woody debris. Also, the annual input of forest litter is dominated by coarse woody debris compared to the earlier stages of forest development, when leaf and fine root debris were the dominant sources of nutrients and organic matter input into the soil.

Some ecosystems may never reach the latter stages of succession if natural disturbances (fire, flooding, hurricanes, etc.) are frequent. A pyric climax refers to an ecosystem that never reaches the potential climax vegetation defined by climate because of frequent fires. The ecotone, a boundary, between grassland and forest is a pyric climax, and only with fire suppression have woodlands and forests began to advance into these regions.

14.Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Succession is a continuous change in the species composition, structure, and function of a forest through time following disturbances.

A.Primary succession occurs at sites where soil must be developed and thus takes a far longer to complete than secondary succession, which occurs where relatively undisturbed soil already exists.

B.The second stage of forest development is much shorter in boreal forests than it is in tropical forests.

C.Old growth forest differs from earlier succession stages in that most soil nutrients come from leaves and fine root debris rather than from dead trees and other coarse woody debris.

D.Early in secondary succession resources are relatively abundant and vegetation increases until canopy closure, after which competition for resources brings about increased plant mortality.

E.With a rising tree mortality, openings in the canopy develop, leading to layered plant growth beneath the overstory canopy, and eventually a climax stage is reached.

F.The effective suppression of grassland wild fires has provided opportunities for the ecotone to proceed into a forest.

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本题为总结题,对同学们的阅读速度和能力都提出很高的要求。下面我们逐一来看选项。A选项定位至文章第一段中的这句话“A longer time is needed to reach the climax sere for primary than secondary succession because soil development must first take place in primary succession.”,故A选项符合原文,正确。B选项定位至文章第三段最后一句“Canopy closure, or maximum leaf area, can occur within several years after disturbance in some tropical forests, but may take three to fifty years in evergreen forests.”意思是在一些热带雨林受干扰之后,林冠郁闭或叶面积达到最大值的情况可能会在几年后发生,但在常绿森林中这个过程可能需要三到五十年。而B选项说常绿森林需要的时间比热带雨林少,明显与原文矛盾,所以B错误。C选项定位至文章第四段最后一句话,提到在森林发展的最后阶段,树木营养主要来自粗木质残体。而在森林发展的早期阶段,叶子和细根残留是土地营养和有机物的主要来源。但C选项完全颠倒文意,故排除。D选项是文章第三段和第四段的总结。文章第三段说,在森林发展第一阶段,光照、营养和水的利用度最高。而第四段说,在森林发展的第二阶段,一些树会因为竞争不到光线、营养和水而死亡。故D选项完全符合文意,正确。E选项定位至文章第四段,第四段中说森林发展的第三阶段,树木死亡使树冠顶端有了缺口,能让更多阳光到达森林的地面,林下的植物重新开始生长,然后达到演替顶级。故E选项符合文意,正确。F选项说有效的灭火能让物种群落交错区(在文中指草原和森林交界处)变成森林。但文章只说,通过灭火,草原和森林才能达到演替的后期阶段。故F选项文中并未提及,排除。

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